Sporulation and Budding

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General characteristics of spores
By wet weight spores generally contain:
 25% protein
 20% fat,
 they have a low water content relative to vegetative
mycelium.
Cell walls of spores are generally not fibrillar, but they are
multi-layered and often contain melanin.
Spores contain all normal mycelial organelles.
Respiratory reserves include:
 lipids
 glycogen
 phospholipids
 polysaccharides that can include sugar alcohols like
Trehalose).
General characteristics of spores
Respiration rates in spores are only 1-4% those of
vegetative mycelium, but the more reserves a spore has,
the longer it will survive.
Fungal spores vary in size, shape and colour.
Fungal spores may be unicellular or multicellular.
For example, conidia produced by Alternaria species are
multicellular.
While dormant they exhibit a low rate of metabolic
activity.
They vary in the primary functions they serve, which may
including dispersal to a fresh site or host, survival at the
same site or increasing genetic variation.
They also vary in the methods by which they are formed,
released and dispersed.
Importance of Fungal Spores
Importance:
 dispersal of
reproduction,
act as a seed
 Survival
Type of spores:
1)Survival spores
2)Dissemination spores
Survival Spores:
Formed in response to:
 Adverse abiotic conditions that can include
desiccation, high UV, high/low temperatures or
starvation.
 Biotic factors can also induce sporulation
including
competition,
antagonism,
and
pathogens presence. These spores have thick cell
walls, and lots of reserves.
Dissemination spores:
Spores that are smaller, with thin cell walls, and
limited reserves
will germinate readily when on a suitable substrate.
Formed as part of the active life cycle of the fungus.
often concerned with epidemic spread of a
pathogenic species from plant to plant, or with rapid
colonization of a substrate.
Dormancy
Definition – time gap between sporulation and
germination (spores do not immediately
germinate after formation).
Aka “break in the life cycle”.
Characteristics – No morphological change
seen. Metabolic rate is slower than during
vegetative growth.
Two types:
1) endogenous (constitutive)
2) exogenous (induced)
Types of Dormancy
1) Exogenous
 Occur because of unfavourable conditions.
 Ex: no conducive temperature or enough food.
2) Endogenous
 Dependence on metabolic characteristics and
spores structures.
 Need special conditions to trigger this type of
dormancy.
 Germination of spores can be stopped even when
conditions are favourable.
Table 1.
Endogenous vs Exogenous Dormancy
Endogenous dormancy
Exogenous dormancy
Definite launch mechanism
Released by autolysis
Small and thin walled spores
Large and thick walled
Short survival time
Survive for a long time
Germinate readily under suitable
conditions
Germinate after a specific stimulus
or removal of an inhibitor
Factors influencing dormancy
1)
2)
3)
4)
Self-stopping spore germination
Mycostasis
Nutrient storage
Wall composition and shape of
spores
1) Self-stopping spore germination
Dense spore suspension and large number of spores
stop spores from germinating.
Examples:
 Uromyces phaseoli (fungus causing rot in beans,
sunflower, corn, snapdragon and groundnuts)
contain a derivative of cinnamic acid that will
stop growth of germ tubes.
 Puccinia graminis tritici (cause rot in wheat stalks)
have germ tubes that grow out of their spore
wall. Germination of this fungus is stopped when
a complex protein is formed and closed these
pores and prevent them from germinating.
 Conidium of Peronospora tabacina become
dormant when a compound called quiesone will
stop its protein synthesis.
2) Mycostasis
Unsterile soil is able to stop germination of fungal
spores and this condition is called mycostasis or
fungistasis.
3) Nutrient storage
Spore has nutrients stored in their vacuoles.
In the conidium of Penicillium, reserved food are
kept as polyol.
Germination can only occur when there is enough
food.
Dormant spores have low metabolic rates and this is
only enough to sustain life.
4)Wall composition and shape of spores
Dormant spores are exposed to unfavorable
conditions.
Spherical spores give minimal surface exposed to the
environment.
Melanine which is a black pigment is a component
of most spores and not easily degraded by
microorganisms.
It therefore gives protection to infection by
microorganisms and also the penetrating power of
the sun’s rays.
Types of Dispersal
1) Usage Of Energy
 Passive Dispersal of Spores: No Own Energy; Use
Environment.
 Active Dispersal of Spores: Use Energy From Fungi.
2) Characteristics of Spores
 Dry Spores
 Slimy Spores
 Wet Spores
Combination
(1+2)
Passive + easily
wetted spores
Passive + Dry
spores
Passive + Easily Wetted Spores
1) Stalked Spore Drop
Stalk is 50µm – 1 mm long.
Transported by insects.
Carried by rain water. Can also
transport dry and slimy spores.
Examples: Sporangia of Dictyostellum
and Mucor; Perithecia of Ceratocystis;
Conidiophore of Cephalosporum and
Graphium.
Stalked spores
Sporangium of Mucor
Synemmata of Graphium spp.
Synemmata are bundles of erect
hyphae and conidiogenous cells
bearing conidia.
Passive + Easily Wetted Spores
2) Rain Splash / Drip Splash / Splash Cup
Rain wet fungi and slime are diluted. Water
droplets carry spores far away.
Peridioles are specific names for splash cups
and spore sacs in some fungi. Peridioles which
are bird’s nest-like are found in Basidiomycetes.
Spores can be transported 1 m away through
this method.
Rain splash / drip splash / splash cup
Peridioles of birds nest fungi
via air and water
Earthstars release their spores in the same way as
puffballs. Amongst the other puffball relatives the
tough skin of Scleroderma splits to expose the
spores to wind and water, allowing the spores to
be washed or blown away.
Earthstar
Scleroderma
Via animals
Truffles
Stickhorn
Passive + Dry Spores
Dry spores dispersed from spore sacs by :1) Mechanical Action
Stream of air form when animal knocks on
vegetation and small spores are released.
When hot air rises and cold air settles. Hot air
carry spores.
Rain can also break spore sacs and release
spores.
Example : Podotaxis have spore sacs
containing elaters that change to changes in
moisture and thereby further dispersing spores.
Passive + Dry Spores
2) Electrostatic Repulsion
There are different charges in fungi and
leaves.
The different charges are results of
changes in moisture and infra red rays.
If charges are similar for example spores
and their spore sacs, they repel each
other. Spores are then released.
Active Dispersal
1)
Turgid Cells Break
When asci mature, ascospores are pushed out
of asci. There is hydrostatic pressure in asci and
ascospores are thrown a few mm away. This will
depend on size of ascospores.
Example : Neurospora, Pyronema, Sordaria,
Claviceps (ASCOMYCETES).
Some spores are released with strong vibration.
Example : Nigrospora (DEUTEROMYCETES),
Pilobus (ZYGOMYCETES).
Active Dispersal
2) Change in Cell Shape
Spores can be dispersed if shape of sacs
changed quickly.
Aeciopores of Puccinia are polyhedral in shape
(7 faces); absorb water and become spherical;
during the change force is produced and throw
spores out of sac.
Example : Spores of Puccinia.
Budding Process
Yeasts buds initiated when
mother cells attain a critical cell
size at a time coinciding with the
onset of DNA synthesis
Followed by localized weakening
of the cell wall together in tension
exerted by turgor pressure →
allow extrusion of cytoplasm in an
area bounded by new cell wall
material synthesized by enzymes
(glucan & chitin synthases).
Chitin,
a
polymer
of
Nacetylglucosamine, forms a ring
at the junction between the
mother cell and the bud.
Budding Process (con’t)
The chitin ring will eventually form the characteristics
bud scar after cell division.
Once new daughter bud has initiated, cell surface
growth during the remainder of the cell division
cycles is restricted to the bud (mother cell wall does
not grow very much during budding.
≡ mother and daughter bud are contiguous during
bud development)
Once mitosis is complete and the bud nucleus and
other organelles (ex: mitochondria) have migrated
into the bud, cytokinesis ensues and a septum is
formed between mother and daughter.
Budding Process (con’t)
A ring of proteins
called
septins
are
involved in positioning
cell division. These
septins are encircle
the neck between
mother and daughter
for the duration of cell
cycle.
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